An air-coupled ultrasonic sensor using a piezoelectric element is utilized for an obstacle detection system for detecting an obstacle or other systems. Such an obstacle detection system transmits ultrasonic waves to air from an air-coupled ultrasonic sensor. The transmitted ultrasonic waves are reflected by an obstacle or the like, and propagates through the air. The air-coupled ultrasonic sensor receives again the ultrasonic waves, to thereby detect the obstacle.
In order to lengthen an ultrasonic wave propagation distance of the air-coupled ultrasonic sensor, it is necessary to increase an output sound pressure of ultrasonic waves. As a method of increasing the output sound pressure, there is known a method of utilizing an acoustic matching layer for matching acoustic impedance of the air-coupled ultrasonic sensor with acoustic impedance of air. An example thereof is an air-coupled ultrasonic sensor in which a low-density region is provided in a case portion of the air-coupled ultrasonic sensor and the low-density region is used as an acoustic matching layer (see, for example, Patent Literature 1).
Patent Literature 1 discloses an air-coupled ultrasonic sensor in which a piezoelectric oscillator is housed in the case portion and an acoustic matching layer is provided on an ultrasonic wave radiation surface of the oscillator. In the air-coupled ultrasonic sensor, a low-density region is formed at a mounting portion of the piezoelectric oscillator at the time of molding the case portion, and the low-density region is used as the acoustic matching layer.
In the air-coupled ultrasonic sensor disclosed in Patent Literature 1, the piezoelectric oscillator is mounted at the case portion, and the mounting portion is the low-density region formed at the time of molding the case portion. The low-density region of the case portion has lower acoustic impedance than in other regions, and hence the low-density region operates as the acoustic matching layer so that the output sound pressure of the air-coupled ultrasonic sensor can be increased. The output sound pressure corresponds to the ultrasonic wave propagation distance. Thus, when the output sound pressure is increased, the ultrasonic wave propagation distance can be lengthened.